If $a , b$ and $c$ are three non-coplanar vectors and $p , q$ and $r$ are vectors defined by $p =\frac{ b \times c }{[ a b c ]}, q =\frac{ c \times a }{[ a b c ]}$ and $r =\frac{ a \times b }{[ a b c ]}$, then the value of $( a + b ) \cdot p +( b + c ) \cdot q +( c + a ) \cdot r$
is equal to